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Port flow limit
A port's flow is tied to its area and how well it is shaped. Enter the port area and a flow factor, and you get an estimated flow in CFM, adjusted for the air temperature via the density. It gives a starting point before the ports are measured on the flow bench.
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Theoretical port flow
291,9 CFM
Theoretical flow at the given flow factor and density (reference 0,075 lb/ft³ at 20 °C). Colder air is denser and increases the flow. Real port flow is measured on a flow bench.
How the calculation works
The base flow is the port's cross-sectional area in square inches times a flow factor in CFM per square inch. The factor describes how well the port breathes: a rough cast port sits lower, a well-shaped race port higher. We convert your area from square millimetres to square inches for you (one square inch is 645,16 square millimetres).
The air density affects the mass flow. We start from 0,075 lb/ft³ at 20 °C and adjust with the ideal gas law, where density is inversely proportional to the temperature in kelvin. Colder air is denser and gives more flow, warmer air thinner and less. The result is quoted at 28 inches of water, which is the flow bench standard.
Example
A port of 1290 mm² (about 2,0 in²) with a flow factor of 146 at 20 °C gives around 292 CFM. At 10 °C the density rises and the flow to about 302 CFM.
Flow factor by port level
| Port level | Flow factor (CFM/in²) |
|---|---|
| Stock engine | 125-130 |
| Mild trim | 135-140 |
| Race engine | approx 146 |
Guide values. The real flow factor varies with port shape, valve and taper and is confirmed on the flow bench.
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